That’s all fine Snuggles, but how should I heal?
As a questing shadow priest, you should spend your time flash healing, popping renew on your tank after the first flash heal and keeping it there. 

PW:S should almost always be reserved to two situations in groups:

1. Oh crap, red warrior is about to die – Your tank just got critted for a bunch and is now red, or a rogue that you are at least partially fond of just got agro and had the snot beat out of them, OR your mage just saved you by popping a critting instant pyroblast on the demon that you just agrod is about to get smushed.
2. The Manick/Snuggles Show – Your mage is AOE’ing and you are flash healing as quickly as you can stab that button.  Pop you shield every chance you get, in fact if you can slow your mage friend down, make him count to 10 after you pop it so you can get a second one quicker.
   This is because Power Word Shield is exceptionally inefficient.  When soloing Shield is part of the Priest killing method (more on that later).
    

Initially during boss fights, the leveling priest should use flash heal, but as they get a handle on the battle, they may switch to chaining greater heal as long as the situation is under control.

If a priest is in a situation where more than 3 or more of their teammates are getting wailed on, Prayer of Healing can be used, two with Improved Prayer of Healing. Prayer of Healing should be followed by a fade if any of the mobs so much as twitches.  Speaking of fade, use it as often as possible.  Generally, greater heal should be avoided unless you have the Master Healer Talent or are very experienced.  Greater heal takes practice to perfect. It can be chained (described in the Master Healer Spec section), though it is more difficult and does not have the massive mana efficiency advantages w/out Improved Healing and Spiritual Healing as these abilities move the mana efficiency from 2.6 to 3.4 Health/Mana.

Once the priest reaches level sixty and has an end game spec, their healing depends on their build.  Most of the standard builds described above show how to heal. Make sure to remember these general rules of healing:
 

1. Think efficiency – Try to use the most efficient heal for the moment.  Do not over heal. When casting Flash Heal, be cognizant of the crit. For instance, end tier Flash heal does 1k health and crits for 1500. Therefore cast it on your tank when they are down 1500 health instead of 1000 so that if the spell crits you can take advantage of it.
2. Don’t waste healing – While it is easy to pop a renew on your rogue that got hit for 400, you waste half the healing potential and thus the mana. Wait until the targets have taken more damage before healing them.
3. Yellow Life Bars are not bad – Keeping your party’s life bars topped off sounds like a great idea, but encourages over-healing, and reduces the effectiveness of your renew spell.   Allowing your teammates life bars to go yellow in controlled situations helps you maximize mana regeneration and lean on your more efficient spells.
4. Casting spells before instants!  -- This is a common mistake.  It is very easy to cast Renew, then Flash Heal, but this costs you two tics of mana regeneration.  Two tics does not sound like  much, but end game, two tics is can be 150-200 mana, which is half a flash heal and can be the difference between life and death.
5. Try to minimize the five second rule by casting a series of spells in a series.  If your tank and off tank are both taking damage try to wait until both need heals before casting.  Again, this gives you more time for mana regeneration.
6. Be Cognisant of Mortal Strike – In the end game Mortal Strike (all healing is reduced by 50%)  is a common debuff used mobs and players alike. Try to time your heals so that they land right after mortal strike (a five second debuff from mobs, 10 from players) finishes. It helps at first to count out loud, but eventually this becomes engrained. If your target must be helped, use PW:Shield, is now more efficient than flash heal and unaffected by Mortal Strike.
7. Renew is a damage mitigation spell not a healing spell for you tank. During combat, keep renew on your tank to provide you more control over the rate of damage received.  Most other classes can be healed with renew when they take incidental damage.
8. Dispell and Cure the debuffs as they come – Most end game debuffs do very bad things, and it is more important to remove them before healing your target.
9. Do not be ashamed to scream for help.  In fact, have a help macro. As the healer you are the most important member of the team, such that if you go down, chance are the team will.  Therefore, it is important to scream like a little school girl when you are getting attacked.
10. Don’t Panic!  Don’t Run – If you get agro, don’t run, in fact move towards your tank.
11. Stand Back – Heals have a range of 40 yards. Take advantage of this to give your team the most time to react when you gain agro.  Try to keep your tank targeted when standing back to ensure that you do not slip out of range by accident. Keep this in mind during Player versus Player combat, where standing back keeps you from becoming target “numero uno”. Many smart rogues in battlegrounds will hang out around you stealthed, waiting for a member of the other team to attack you.
12. Be cognizant of surrounding – Stairs, walls, columns, trees, rocks, small children, and other pieces of the environment get in the way of your healing, even if it looks like you can see. Pay attention to these elements, and if necessary, direct your team to move back from them.
13. If you puller has a bad pull that they stand no chance of recovering from, stand their and do nothing, or run – This is the hardest thing to learn, but essential to succeeding in high end instances. As long as you do nothing, mobs will not agro on you. It is a tough lesson, but it is easier to rez your teammates than take the long walk and deal with re-pops. It is important to encourage this in your group…if the tank accidentally pulls 2 groups instead of one, you have a very small chance for survival. Resurrection is designed for this situation, has a 40 yard range, and resurrects them where the priest is.  While this might cause some bad blood in PUGs, good players will recognize what you are doing.  Some high end instances (Scholo) are practically set up for running, and your team will appreciate you healing/shielding them on the way out, just remember that you are the important person to escape.
14. Psychic Scream in instances is not always bad, just most of the time – Doing it is either a last resort, or in a specific situation (the Three Fire Elemental Pull in Black Rock Depths).  Used correctly, the group will survived.  Used incorrectly is a guaranteed way to wipe your group.  In any case, the use of Scream can be exceptionally powerful and pull your group out of sticky situations.
15. This goes hand in hand with #14. Always encourage your group to pull back into a previously cleared room.  In addition to lowering the likelihood of the group picking up adds, this also allows the priest to use fear (in a controlled way) if the situation gets out of hand.  In a raid, two shadow priests or three holy priests using their fears in concert, can keep mobs (or enemies) feared for a virtually unlimited amount of time (until diminishing returns sets in).
16. When leaving an instance ALWAYS prep Psychic Scream (mash the hotkey as you load). This will give you a split second to react if you leave an instance and there are enemies at the entrance.  Chances are, you and your group are dead, but this gives you a chance to react.
     

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